1. Field of the Invention
The present invention relates to an exterior structure for a keyboard instrument, which includes a case structure of a multi-case coupled type for mounting a keyboard unit therein.
2. Description of the Related Art
Conventionally, as disclosed e.g. in the publication of Japanese Utility Model Registration No. 2545353, an exterior structure for a keyboard instrument is known in which two upper case of respective types are selectively combined with one lower case of a single type, thereby enabling two types of case structures to be constructed. In general, in the exterior structure of this kind, the upper cases and the lower case are made of resin and shaped using molds. Screw pilot holes are formed in each upper case, and clearance holes for screwing screws are formed in the lower case in a manner associated with the screw pilot holes. Then, the two cases are secured to each other by screwing the screws from below.
In this type of exterior structure, however, when the angle of the panel surface of each upper case is desired to be changed according to the type of keyboard instrument, for example, to prevent undercuts from being formed in portions of a mold associated with the screw pilot holes, it is desirable that the screw pilot holes in the upper case are formed to be approximately perpendicular to the panel surface. However, when a screw pilot hole is formed to be perpendicular to the panel surface, it is also desirable that the angle of a surface of the lower case, with which the head of a screw is brought into contact, is set according to the angle of the screw pilot hole (such that the surface of the lower case becomes perpendicular to the screw pilot hole). This makes it difficult to share or commonly use the lower case between the different types of keyboard instruments.
On the other hand, it is contemplated that the screw pilot holes in the upper cases are formed to be elongated in cross section perpendicular to the axis thereof such that screws can be screwed obliquely into the elongated screw pilot holes, to thereby enable a large number of upper cases of respective types to be selectively assembled to one lower case of a single type. In this case, however, each screw mates with only parts of the elongated screw pilot hole along shorter sides thereof to form an exposed part, which degrades fastening rigidity of the screw. Further, the screw is difficult to advance straight during screwing, which degrades fastening accuracy thereof. Moreover, an elliptic projection associated with the elongated hole has to be formed on the mold, and hence it becomes more difficult to machine the mold than when a hole circular in cross-section is formed, which results in an increase in the cost of the mold.
Further, when a lower case of one type is commonly used or shared between various types of keyboard instruments, even if a plurality of upper cases are provided, the depth of each keyboard instrument and the shape thereof in plan view are determined depending on the lower case, and hence from the viewpoint of design and installation stability thereof, the depth of the upper case in particular cannot be increased, which makes it difficult to manufacture keyboard instruments different in size between different types thereof.
For the above-described reasons, if it is a prerequisite to reduce the cost of molds, there are significant restrictions imposed on the design of upper cases, and in actuality, it is difficult to cope with production of various types of keyboard instruments by sharing the lower case and changing only upper cases on an instrument type-by-instrument type basis. Therefore, the fact is that the number of types of keyboard instruments that can be manufactured is limited.
Further, in the exterior structure of the above-mentioned kind, the upper and lower cases are originally formed as separate members, so that unless portions thereof via which the two cases are assembled are deliberately configured, there is a fear that the rigidity of the whole case structure decreases. Further, if the case structure is constructed by combining three or more separate case members, there is an increased fear that the rigidity of the whole case structure decreases. Furthermore, when a keyboard unit demanding high mounting accuracy is mounted in the case structure, whether or not the accuracy can be maintained largely depends on the structure of portions of the upper and lower cases via which they are assembled to each other, and portions of the same via which the keyboard unit is mounted.